For a variety of different applications, it is desirable to coat a surface with a paint that has specified reflective properties for some wavelengths of light while it remains transparent or substantially transparent to others. For instance, bird strikes are a significant problem for aeronautical applications, and in the most severe cases have led to well-documented crashes. See, N. S. Sodhi, “Competition in the Air: Birds Versus Aircraft,” The Auk 119, 587 (2002) and “Wildlife Strikes to Civil Aircraft in the United States 1990-2013,” Federal Aviation Administration National Wildlife Strike Database, Serial Report #20 (2014). These situations arise, in part, from the fact that birds use ultraviolet (UV) light to detect obstacles and most man-made flying objects have low reflectivity in the UV spectrum. A paint that reflects UV light that could be applied to an aerial vehicle's body, windows, and engine parts without significantly changing the visible appearance of the vehicle could reduce the likelihood of bird strikes and therefore improve flight safety. It has been previously disclosed in, for example, U.S. Patent Publication No. 2010/0098844A1 that UV reflective coatings on a variety of surfaces could enhance both human and bird safety.
Furthermore, a large number of birds die as a result of flying into windows on commercial and residential buildings. The ecological impact of this situation is huge, with estimates placing the number of bird deaths resulting from window collisions in the range of 365 to 988 million per year in the United States alone. See, S. R. Loss, T. Will, S. S. Loss, and P. P. Marra, “Bird-Building Collisions in the United States: Estimates of Annual Mortality and Species Vulnerability,” The Condor 116, 8-23 (2014). A paint for windows that provides UV reflectivity while leaving the window visibly transparent could reduce the number of these collisions. One method has been proposed that uses a UV reflective structure embedded in the glass, see for example, “Ornilux Bird Protection Glass,” http://www.ornilux.com/Attachments/ORNILUX_Flyer_2014.pdf. However, this technology is costly and cannot be used to retrofit existing windows.
Paints with controlled reflectivity could be useful for a variety of other applications as well. They could, for instance, be applied as decorative coatings. Traditional paints reflect light in one or more spectral bands, resulting in their color. However, there are instances when more complex spectral properties are desired. For instance, a paint could be made to reflect a specific color for a certain angular range, adding an interesting visual effect that could be described as a “sparkle” or “glitter”. Alternately, a coating that offers high reflectivity over a controlled band could provide a higher degree of color purity than traditional paint, providing, for example, a coating that appears to be a brighter red than ordinary red paint.
Paints containing reflective flakes have been disclosed previously. For example, see U.S. Pat. Nos. 7,608,330, 5,059,245, 5,084,352, 5,135,812, and 5,383,995. These rely on vacuum deposition onto a “web,” which is a substrate layer. These methods are best suited for coatings that require a single or a few layers. For more complex coatings that require many layers and are thus thicker, vacuum deposition on a web is impractical because it is slow and costly. It has been previously disclosed that a polymer stack may be designed to reflect UV light, see U.S. Pat. No. 5,540,978. Such a stack can be engineered to have reflectivity at a desired wavelength, transmissivity at others, potentially with these properties varying with incident angle.
The present invention utilizes aspects of technologies previously known and to provide a novel coating that comprises flakes of polymer films, suspended within a paint matrix. The films are obtained from multilayer polymer stacks or films that use thin film interference in order to achieve the desired reflective properties. Alterations to the present coating as discussed in detail below are able to provide a wide variety of reflective properties.